Stimulus response coupling mediated by calcium and calmodulin involves several steps. (1) The calcium release from the endoplasmic reticulum induced by inositol triphosphate which has been shown to be modulated by intracellular calcium levels. (2) The binding of calcium to calmodulin is a cooperative and selective process that is modulated by magnesium. At physiological ionic strength, and only in the presence of magnesium, a large difference is seen between the affinities of sites III and IV (0.16 and 0.24 x 10-6-M-1), and sites I and II (0.01 and 0.07 X 10-6-M-1) for calcium. The large negative cooperativity between the calcium sites located in the two halves of the calmodulin molecule, induced by magnesium, requires the integrity of the calmodulin molecule. The magnesium-induced difference in affinity of the calcium binding sites, together with the positive cooperativity, explains the stepwise conformational changes induced by calcium. (3) The interaction of calmodulin with its target proteins involves different portions of the calmodulin molecule. Calmodulin-regulated enzymes can be divided into three classes according to their abilities to bind and to be activated by calmodulin fragments: (a) enzymes which are activated by the C-terminal fragment include the Ca2+ ATPase, phosphorylase kinase and B. pertussis adenylate cyclase; (b) enzymes which require both halves of the molecule such as cyclic nucleotide phosphodiesterase and myosin kinase; and (c) enzymes whose interaction with calmodulin fragments is too weak to be detected by activation, such as the calmodulin-regulated protein phosphatase, calcineurin and the multiprotein kinase. Thus, different enzymes may be activated by different calmodulin conformers and the stepwise changes exhibited by calmodulin at different calcium levels can be used to regulate different metabolic pathways. Stimulus-response coupling by cAMP are also being studied by J. Foster who has cloned the gene of the catalytic subunit of cAMP-dependent protein kinase in Drosophila melanogaster.